signals and copy constructors

[Antonio Coralles <noche suapie reflex at>]

i know that this is more a libsigc++ question; but as gtkmm uses 
libsigc++ heavily and the libsigc++ mailinglist is not very populated i 
ask this question here too:

is there an elegant way to "change  *p_obj in  mem_fun(*p_obj, ...)" 
after the signal is allredy attached to a concrete object , so that when 
the copy constructor of that object is called,  some_signal.emit(...) 
calls the method in the object created by this copy constructor call ? 
or is it better to think of a different design, so that this question 
doesn't come up ?

to avoid misunderstandigs, here is somekind of example:

//just a useless signal
sigc::signal<void> someSignal;

//just a useless class
class Useless : public sigc::trackable
{
   public:
   void call_me() { ... }
};

//a useless scope
....
{
   Useless u1;
   someSignal.connect(sigc::mem_fun(u1,  &Useless::call_me));
  //ok now connected - but how can i automatically change the target 
object to u2 - so that u2.call_me() is called due someSignal.emit() ?
  Useless u2(u1);  //imagine this call is made at a comletly different 
part in my programm - for example in a clone method ....
  ....
}
....